Angioplasty procedures have gained wide acceptance in recent years as efficient and effective methods for treating types of vascular disease. In particular, angioplasty is widely used for opening stenoses in the coronary arteries although it is also used for the treatment of stenoses in other parts of the vascular system.
The most widely used form of angioplasty makes use of a dilation catheter which is threaded over a guidewire and has an inflatable balloon at its distal end. Inflation of the balloon at the site of the occlusion causes a widening of the lumen to reestablish an acceptable blood flow through the lumen.
Often it is desirable to determine the severity of the occlusion in order to properly choose a dilation catheter. Various invasive techniques have been used to determine the severity of the occlusion. One way of determining the severity of the occlusion is to measure pressure both proximal to and distal of the stenoses. Devices that are used for this purpose include catheter-like members with some type of pressure sensing device incorporated therein. For devices of the single pressure sensor type, movement of the catheter-like member must be accomplished to measure pressure at different locations. Whereas, devices with more than one pressure sensor can measure pressure simultaneously at several points.
One known device measures the pressure as a function of the deflection of a diaphragm located at the distal end of the catheter. Positioning the sensing part of the sensing device at the distal end of the catheter requires the sensing device to be made extremely small. Otherwise, the sensing device will impede blood flow and effect the pressure reading. In addition, catheters with the sensing device at the distal end do not allow for reuse of the expensive sensing device.
Another known device, called a fluid filled catheter-manometer, connects a sensing device to the proximal end of the catheter. A catheter-manometer uses a fluid column that can communicate pressure changes at the distal end of the device to a transducer located at the proximal end of the device. A catheter-manometer has the advantage of having a reusable sensor and is therefore less expensive. Unfortunately, catheter-manometers have heretofore not been accurate at physiologic frequencies. The required bandwidth, or flat frequency response, for accurate physiologic pressure measurement in the heart about 30 hertz. Large catheter-manometers (8 Fr) are typically only 20 hertz and small catheter-manometers, like guidewire systems (0.014"), may be less than 1 hertz. Further the bandwidth of catheter tip pressure devices may be several kilohertz. Clearly, prior art catheter-manometer systems have not had the required bandwidth or frequency response for accurate physiologic pressure measurement.
There are several factors causing prior art catheter-manometer systems to not have the required bandwidth or frequency response for accurate physiologic pressure measurement. One factor is that a relatively large amount of fluid mass must be moved with a relatively small amount of pressure. The movement of the mass is described by Newton's second law of motion: F=Ma, where the force F is a combination of a displacing force and restoring force. The displacing force is the pressure input and the restoring force is the natural rebound response of the system. Therefore, the density of the fluid becomes a contributing factor to the frequency response.
Another factor is resistance R. The resistance to flow in a catheter can be described by Poiseuille's equation: R=8.mu.L/.pi.r.sup.4, where r is the internal diameter and L the length of the catheter. In a fluid-filled manometer system the fluid is typically not flowing, but rather is oscillating, as originally described by Lambossy (1952). Thus, in a fluid filled manometer, the damping (resistance) varies as the square-root of the length and inversely as the cube of the radius as follows: ##EQU1##
Yet another factor is the compliance or elasticity E of the system. The catheter combined with the pressure transducer form an elastic volume in which the response is described by: ##EQU2##
Thus, the natural frequency response of the fluid filled catheter manometer will be determined by stiffness or compliance of all components in the system. Prior art systems are composed of the catheter, a manifold, tubing, fittings, and a transducer. Catheters are made of combinations of metals and plastics which may, depending on construction, have compliance which absorbs some of the pressure change. Manifolds are made of hard plastics which typically do not elastically deform. However, the tubing used is elastomeric, as are the gaskets used with the fittings. These elastomers are very flexible and will dampen pressure change in the fluid passing through them as described above.
Finally, the responsiveness of the pressure transducer within the sensing device is also a factor. As previously described, all components in the system contribute to the elasticity E, and thus the frequency response. In order to have adequate sensitivity in the physiologic pressure range (about 0-5 Psi), blood pressure transducers have traditionally been designed for full scale signal output (maximum pressure rating) at applied pressures of 300 mmHg or 5 Psi. The very flexible diaphragms used to create adequate low pressure sensitivity increases the elasticity of the system, and thus, lowers the frequency response. All of these factors have thus far contributed to making devices of this type less accurate in reproducing the exact physiologic signals and produce pressure readings which are, in some devices, only an average of a patient's blood pressure over time.
In addition to procedures like angioplasty where it is desirable to know the pressure at a given point at a given time, it is also highly desirable to know the pressure wave form for places within the heart and various vessels. Standard pressure wave forms have been documented for the healthy vasculature. A, variety of other wave forms have also been documented to correspond to particular maladies. Therefor, a device which was responsive enough to measure wave forms within the heart chambers would also be useful.
Tremulis discloses in U.S. Pat. No. 4,953,553, U.S. Pat. No. 4,964,409, and U.S. Pat. No. 5,050,606 guide wires capable of sensing pressure via a proximal sensing device. This device has the added advantage of being usable as a guidewire. Guide wires are commonly used during angioplasty as well as many other procedures. However, this device is not as sensitive as distal sensing devices because of all of the factors previously described nor does it have enough frequency response to measure accurate wave forms.
Accordingly, it is desirable to provide a device that can be used as a guidewire and simultaneously be used to measure pressure at specific points without the added expense of distal sensors and without the pressure change insensitivity of prior art proximal sensing devices. Further it would be highly desirable to combine this device with a device that could produce a true wave form of the changes in blood pressure as well as an average blood pressure measurement.